The invention relates to a color display tube comprising an electron gun, a display window with a screen, and a color selection electrode having a shadow mask positioned between the electron gun and the screen, said shadow mask, having a center point, a horizontal axis and a vertical axis intersecting at the center point, is provided with a hexagonal pattern of round apertures which are arranged on substantially horizontal and vertical lines.
The invention further relates to a shadow mask for use in such a color display tube.
A color display tube as described in the opening paragraph is generally known. Shadow masks with a pattern of more or less round apertures arranged in a hexagonal structurexe2x80x94also referred to as xe2x80x98dotted typexe2x80x99 shadow masksxe2x80x94are commonly used in cathode ray tubes for displaying data, like for instance computer monitors.
This kind of color display tube often shows a worsened landing performance, due to the geometrical construction of such a tube. The dots of one triad tend to be rotated on the screen. A triad is a set of three dots containing the three different colors of phosphor corresponding to one aperture in the shadow mask. Due to this phenomenon, which is referred to as triad rotation, the landing of the color display tube becomes more critical and consequently, the color purity performance is worse.
In principle this triad rotation problem, which is most significant in the corners of the display screen, can be solved by giving the horizontal lines a shape that follows the amount of triad rotation. As a result the horizontal lines should be barrel-shaped. This is disclosed in U.S. Pat. No. 3,721,853, resulting in a color display tube with an improved color purity.
However, the solution as described in U.S. Pat. No. 3,721,853 brings about another problem: due to the barrel-shaped horizontal lines, the distance between the horizontal linesxe2x80x94that is, the vertical pitchxe2x80x94varies quite largely over the entire screen leading to moirxc3xa9 problems.
It is an object of the invention to obviate this drawback by providing a color display tube with an improved shadow mask which properly solves the triad problem and also has an excellent moirxc3xa9 performance.
According to the present invention, this object is achieved by means of a color display tube which is characterized in that the apertures on a horizontal line follow a curved trajectory which is a polynomial of order 2n that can be described by   y  =            y      0        ·                  ∑                  k          =          0                n            ⁢                        a                      2            ⁢            k                          ·                  x                      2            ⁢            k                              
in which x is the horizontal coordinate and y is the vertical coordinate measured with respect to the center point, y0 is the intercept of the horizontal line (21) with the vertical axis, a0=1 and a2k are constants for 1xe2x89xa6kxe2x89xa6n at least one of which is unequal to zero.
The invention is based on the insight that by using a polynomial of order higher than two, it is possible to obtain in the corners the same amount of inclination of the horizontal lines but the positional difference will be smaller. This is in contrast to the prior art shadow masks having a pure parabolic shape of the horizontal lines. This is a direct consequence of the general nature of polynomial functions. As a result, the difference in vertical pitch between the center region and the edge region will be smaller. This measure enables a design of the shadow mask that fully compensates the triad rotation in the corners, in combination with a much smaller deviation of the vertical pitch, which is very beneficial to the moirxc3xa9 performance of the color display tube.
In practice, in most cases a satisfactory solution will be reached with a horizontal line shape that is characterized in that y=y0.(1+a2.x2+a4.x4+a6.x6) and at least a4 or a6 is unequal to zero.
The direction of the triad rotation is such that the vertical pitch at the vertical edges is smaller than on the vertical axis. This is achieved by making the horizontal lines barrel-shaped. This means that the first derivative of the formulae for the horizontal line should fulfil the condition 2.a2+4.a4.xmax2+6.a6.xmax4xe2x89xa60, in which xmax is the x-coordinate at the end of the horizontal axis.
In a preferred embodiment, y=y0.(1+a4.x4). A horizontal line the deviation of which is a pure fourth order term causes the deviation of the vertical pitch to be improved by a factor of two as compared to the parabolic term, while the inclination for opposing the triad rotation remains the same.
In a further embodiment, the improvement with respect to the deviation of the vertical pitch can be increased to a factor of three by using a pure sixth order term in the formulae for the horizontal line shape, that is to say, the horizontal line shape is given by:
y=y0.(1+a6.x6). 
The invention further relates to a shadow mask for use in such a color display tube.
These and other aspects of the invention are apparent from and will be elucidated by way of non-limitative examples with reference to the drawings and the embodiments described hereinafter.